1. Field of the Invention
The present invention relates, in general, to a diffractive micromirror and a method of producing the same and, more particularly, to a diffractive micromirror, in which a recess having a desired depth and width is precisely formed in a piezoelectric operation manner, and a method of producing the same.
2. Description of the Prior Art
Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner, unlike a conventional digital information processing technology, in which it is impossible to process a great amount of data in real-time, and studies have been conducted on the design and production of a binary phase-only filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
Of these, the spatial light modulator is applied to optical memory, optical display, printer, optical interconnection, and hologram fields, and studies have been conducted to develop a display employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended on an upper part of a substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on an oxide spacer layer 12 by a nitride frame 20.
In order to modulate light having a single wavelength of λo, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λo/4.
Limited by a vertical distance (d) between a reflective surface 22 of each ribbon 18 and a reflective surface of the substrate 16, a grating amplitude of the modulator 10 is controlled by applying a voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 of a lower side of the substrate 16 acting as a second electrode).
In its undeformed state, with no voltage application, the grating amplitude is λo/2, and a total round-trip path difference between light beams reflected from the ribbon and substrate is one wavelength λo, and thus, a phase of reflected light is reinforced.
Accordingly, in its undeformed state, the modulator 10 acts as a plane mirror when it reflects light. In FIG. 2, a reference numeral 20 denotes incident light and reflected light in its undeformed state.
When a proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to be moved downward toward a surface of the substrate 16. At this time, the grating amplitude is changed to λo/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 resulting from the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in a +/− diffractive mode (D+1, D−1) in a deformed state.
However, the Bloom's light modulator adopts an electrostatic method to control a position of the micromirror, which has disadvantages in that an operating voltage is relatively high (usually, 30 V or so) and a correlation between the applied voltage and displacement is not linear, resulting in poor reliability in the course of controlling light.
FIG. 4 is a cross-sectional view of a recess-type thin-film piezoelectric light modulator according to a conventional technology.
Referring to FIG. 4, the recess-type thin-film piezoelectric light modulator according to the conventional technology includes a silicon substrate 401 and elements 410.
In this regard, the elements 410, which have predetermined widths and are arranged at regular intervals, constitute the recess-type thin-film piezoelectric light modulator. Additionally, the elements 410 may be spaced apart from each other at regular intervals (each interval is almost the same as the width of each element 410), in which a micromirror layer formed on an upper side of the silicon substrate 401 reflects incident light to diffract it.
The silicon substrate 401 has a recess to provide an air space to each element 410, an insulating layer 402 is deposited on an upper surface of the substrate, and ends of the elements 410 are attached to upper sides of a wall of the recess.
The elements 410 each have a rod shape, and lower sides of ends of the elements are attached to the remaining upper side of the substrate 401 except for the recess so that the centers of the elements are spaced from the recess of the silicon substrate 401. Additionally, each element 410 includes a lower supporter 411 which has a vertically movable portion corresponding in position to the recess of the silicon substrate 401.
Furthermore, the element 410 is laminated on a left end of the lower supporter 411, and includes a lower electrode layer 412 for providing a piezoelectric voltage, a piezoelectric material layer 413 which is laminated on the lower electrode layer 412 and shrunken and expanded when a voltage is applied to both sides thereof to generate upper and lower driving forces, and an upper electrode layer 414 which is laminated on the piezoelectric material layer 413 and provides a piezoelectric voltage to the piezoelectric material layer 413.
Furthermore, the element 410 is laminated on a right end of the lower supporter 411, and includes a lower electrode layer 412′ for providing a piezoelectric voltage, a piezoelectric material layer 413′ which is laminated on the lower electrode layer 412′ and shrunken and expanded when a voltage is applied to both sides thereof to generate upper and lower driving forces, and an upper electrode layer 414′ which is laminated on the piezoelectric material layer 413′ and provides a piezoelectric voltage to the piezoelectric material layer 413′.
Additionally, Korean Pat. Application No. P2003-077389 describes an extrusion type as well as the recess type, and a method of producing the same in detail.
FIGS. 5a to 5j illustrate fabrication of a recess-type thin-film piezoelectric micromirror according to a conventional technology.
Referring to FIG. 5a, a mask layer 502 is formed in a thickness of 0.1–1.0 μm through a thermal oxidation process on a silicon wafer 501, and then patterned for silicon etching.
With reference to FIG. 5b, the silicon is etched using a solution capable of etching the silicon, such as TMAH or KOH, in a predetermined thickness, and the mask layer 502 is then removed.
Referring to FIG. 5c, an insulating and etching prevention layer 503 is formed on the etched silicon according to the thermal oxidation process. That is to say, the insulating and etching prevention layer 503, such as SiO2, is formed on a surface of the silicon wafer.
Referring to FIG. 5d, a polysilicon (Poly-Si) or an amorphous-Si is deposited on an etched portion of the silicon wafer 501 according to low pressure chemical vapor deposition (LPCVD) or plasma chemical vapor deposition (PECVD) processes to form an air space to form a sacrificial layer 504, and the resulting silicon wafer is polished to flatten a surface thereof. In this respect, in the case of using a silicon on insulator (SOI), the deposition of the polysilicon and polishing may be omitted.
Subsequently, silicon nitrides, such as Si3N4, are deposited in a preferable thickness of 0.1–5.0 μm according to the LPCVD or PECVD processes, and SiO2 is deposited in a thickness of 0.1–5 μm according to thermal oxidation or PECVD processes, but this procedure may be omitted according to necessity.
Referring to FIG. 5e, a lower supporter 505 for supporting the piezoelectric material is deposited on the silicon wafer 501, and a material constituting the lower supporter 505 may be exemplified by Si oxides (e.g. SiO2, etc.), Si nitrides (e.g. Si3N4, etc.), ceramic substrates (Si, ZrO2, Al2O3 and the like), and Si carbides. The lower supporter 505 may be omitted, if necessity.
Referring to FIG. 5f, a lower electrode 506 is formed on the lower supporter 505, examples of material for the lower electrode 506 may include Pt, Ta/Pt, Ni, Au, Al, RuO2 and the like, and the material is deposited in a thickness of 0.01–3 μm using sputtering or evaporation processes.
Referring to FIG. 5g, a piezoelectric material 507 is formed in a thickness of 0.01–20.0 μm on the lower electrode 506 according to a wet process (screen printing, sol-gel coating and the like) and a dry process (sputtering, evaporation, vapor deposition and the like). Additionally, all of the upper and lower piezoelectric materials and left and right piezoelectric materials may be used as the piezoelectric material 507, examples of the piezoelectric material may include PzT, PNN-PT, ZnO and the like, and the piezoelectric electrolytic material contains at least one selected from the group consisting of Pb, Zr, Zn, or titanium.
Referring to FIG. 5h, an upper electrode 508 is formed on the piezoelectric material 507, a material of the upper electrode may be exemplified by Pt, Ta/Pt, Ni, Au, Al, and RuO2, and the upper electrode is formed in a thickness of 0.01–3 μm using the sputtering or evaporation processes.
Referring to FIG. 5i, a micromirror 509 is attached to the upper electrode 508, and examples of a material of the micromirror include a light-reflective material, such as Ti, Cr, Cu, Ni, Al, Au, Ag, Pt, and Au/Cr.
At this time, the upper electrode 508 may be used as the micromirror, or a separate micromirror may be deposited on the upper electrode 508.
Referring to FIG. 5j, after such a mother body of a diffractive thin-film piezoelectric micromirror array is patterned using a mask layer, such as a photoresist, the micromirror 509, upper electrode 508, piezoelectric material 507, lower electrode 506, and lower supporter 505 are etched to form the diffractive thin-film piezoelectric micromirror array. Subsequently, the sacrificial layer 504 is etched using XeF2 gas.
Heretofore, there has been described removal of the sacrificial layer 504 after the diffractive thin-film piezoelectric micromirror array is formed from the mother body of the diffractive thin-film piezoelectric micromirror array, but the micromirror array may be formed after the sacrificial layer 504 is removed.
In other words, a hole is formed in a portion of the mother body of the diffractive thin-film piezoelectric micromirror array, in which the lower supporter 505 is not formed, the sacrificial layer 504 is etched using XeF2 gas, the mother body of the diffractive thin-film piezoelectric micromirror array is patterned using the mask layer, such as the photoresist, and the micromirror 509, upper electrode 508, piezoelectric material 507, lower electrode 506, and lower supporter 505 are etched to form the micromirror array.
Meanwhile, the conventional diffractive thin-film piezoelectric light modulator is problematic in that it is difficult to control a thickness in the course of polishing the polysilicon. That is to say, a dispersion of a setting time for each ribbon in a chip is undesirably high.
Furthermore, the conventional diffractive thin-film piezoelectric light modulator is disadvantageous in that a depth of a cavity is ±0.5 μm, and it is impossible to control the depth of the cavity to 0.5 μm or less.